It is the aim to develop methods that can be used for studying neuronal interrelationships with the high voltage electron microscope. Staining methods will be developed so that details of synaptic relationships can be studied in sections that are ten to twenty times as thick as the sections used for conventional electron microscopical studies. Modifications of the Golgi methods and of methods of intracellular staining will be used to label individual nerve cells. Other methods of staining will also be investigated, including modifications of conventional electron microscopical staining methods and methods of marking extracellular spaces with substances such as lanthanum or ruthenium red. Tissues to be studied initially (leech ventral nerve cord, lobster abdominal ganglion, mammalian dorsal lateral geniculate nucleus, cerebral and cerebellar cortex) are chosen because relevant aspects of the anatomy are known, and because we have previous experience with these tissues with conventional e.m. methods.